ANSWER:
The sin is so major and heinous, there is no kaffarah that can accommodate it! A person who does this is cursed by Allah.
https://islamqa.info/en/answers/52803/is-there-any-expiation-for-having-intercourse-with-a-woman-in-her-back-passage
https://islamqa.info/en/answers/52803/is-there-any-expiation-for-having-intercourse-with-a-woman-in-her-back-passage